Cats?
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Cats?
I see a lot of conversation in here about cats; I understand that cat is short for catalytic converter, but the plural cat(s) is confusing me. How many catalytic converters does the Expe have? I thought it was just one.
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Ok. I stand corrected, after my first post I went out and looked under my Expe and saw 4 cats? I think I saw a small and large cat on each side of the vehicle. Is this right? if so, why is there two on each side?
Cross-Country
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because man, the man is trying to save the earth and the whales and stuff.............backpressure.........sell more O2 sensors...........increase the odds of a failure and make you buy more parts...............sorry what was the question?
Cross-Country
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sorry, in an off mood this evening. I don't know the exact answer, but it probably has to do with engineers thinking it was the bset setup for performance, mileage, emissions control, etc. Seems to work ok, i put headers and a catback exhaust on my 97 and had no problems. Going to do exhaust work on my 03 when I get home from the sandbox.
Cross-Country
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it will cause trouble codes to pop up and negatively affect your engine power because of reduced backpressure. If you have the cash I THINK you can get a bolt on high flow convertor setup. If not you can buy high flow cats from Magnaflow and have a shop weld them in.
Just in case you were unsure "catback" means the exhaust system from the catalytic convertor section back. It includes the exhaust pipe that bolts up to the convertor section, the muffler and the tail pipe.
Just in case you were unsure "catback" means the exhaust system from the catalytic convertor section back. It includes the exhaust pipe that bolts up to the convertor section, the muffler and the tail pipe.
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Cross-Country
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Technically I am right, though I should have specified "Torque" and only at lower RPM. The difference is not huge and some drivers may not notice. Here is a link to and a paste of a really good article by.....Michael Lannon.
Homepage of Michael R. Lannan, Milwaukee School of Engineering
This is how I've always understood it also....if I'm still wrong, sorry.
BACK PRESSURE
Hey yall. I've been asked this question on a number of occasions. Why does backpressure effect torque? Which is usually accompanied my the generalization that low backpressure reduces torque. Well, I figured an article was in order after the last time it was asked.
Your assumption of the effect of backpressure is the same as most peoples. That is, that having less backpressure decreases torque and increases horsepower. Likewise, having more backpressure increases torque and decreases horsepower.
I've heard this from countless people, and I always try to convince them otherwise, usually to no effect. So I'll try with you. Hopefully I can explain this well enough. The real crux of all this is at the bottom. But read the article carefully. There is a serious flaw in the logic of this assumption about torque. The rule is generalizing on a level that almost seems greedy.
As a general rule that assumption is correct for the lower revs of an engine. That is, if you took a standard engine, and had nothing more than a set of headers on it, you would find that the low-end torque is somewhat less than that of the same engine with some fairly restrictive mufflers. Likewise, the engine's torque, at bigger revs, would be higher than that of the same engine with mufflers. For conversation's sake, I'll consider low-end to be anywhere from 0-1,800rpm, mid-range to be about 1,800-3,000, and high-end to be 3,000rpm and greater.
I'm not sure where people have come up with the idea that there is a counter-productive variation in horsepower by opening up the exhaust system. I've never come across any studies that prove that horsepower is adversely effected by open exhaust. Not that that is what you claimed; but again. I've heard it from several people.
What it all comes down to is what range your looking at. If you want big horsepower and torque; it's going to have to be with an open exhaust. There is no way to really look at it other than that. The more air you can remove, the more air you can take in. More air means more power. And engine with opened exhaust will always make more peak horsepower and torque than a restricted engine. It will just be at a higher revs.
Exhaust modifications are done purely to alter the mass of air that can exit the engine (duh). Here's a story for you. A friend of mine put a new, uncorked, exhaust system on his truck. The engine is an otherwise stock, injected, small block. He put the exhaust on, and his fuel efficiency went way down, torque went down the drain, and his horsepower was basically non-existent. "But I don't get it. I was supposed to get 10hp out of that."
Why did that happen? Because the engine was outside of it's stochiometric ratio. What is that? That's the chemically "neutral" balance of air/fuel ratio. It has been shown that engines produce maximum horsepower at 14.7:1 (air/fuel) by mass. The problem is that when you alter the backpressure of the exhaust system, the ratio will also be altered. If you reduce backpressure, the engine is allowed to flow more air out. More air out means more air in. If the fuel system is not compensated for the increased air mass, the engine runs lean. If the backpressure is increased, the mass of air leaving is decreased. Less air out means less air in. Again, if the fuel system is not calibrated for the change, the engine will run rich.
As far as normal engines are concerned, alterations on the order of 1.3 parts of air to fuel would result in significant loss of performance. And yes, that's loss; no matter which way it goes. Contrary to popular beliefs, lean engines do not produce more horsepower and/or torque than neutral. So taking an engine, tuned for a specific amount of backpressure, and altering it, would result in the engine running outside of its stochiometric range. The engine would thereby run substantially lean, and be running quite poorly.
Now note that I said it was an injected small block. You might be thinking "well doesn't the ECM calibrate itself for the differing air mass?" Yes, it would. To an extent. Once you start getting somewhere in the range of about 1.5:1 outside of neutral, ECMs are typically unresponsive. As a general rule a code would be thrown and the ECM would just swing to it's maximum ability.
To most people's surprise, the "flowability" of an exhaust system is determined more by its exhaust manifold than the mufflers that are installed. Keep in mind that headers, of whatever style, are nothing more than exhaust manifolds. It is possible to have a low restriction exhaust that boosts low-end torque. That is truly what is wanted if you're looking for low-end torque. Since the backpressure isn't the "direct" cause of the situation at hand; but rather an indirect cause. Keep reading and you'll see what I mean.
A common street-stock exhaust system has about 15-20psi of parasitic pressure loss. Compare that to a typical muffled performance system of about 5psi of parasitic loss. A tube header and extension pipe with no baffling or resonators (an open header system) would have about 2psi of loss.
A street-stock system is one in which all the usual stock equipment is on the vehicle. That is a cast style exhaust manifold, reverse-flowing mufflers, and standard diameter tubing with heavy bend kinks. A performance system is one where the engine employs tubular headers, larger exhaust tubing (usually) and a perforated straight-through baffle muffler. Notice I did not say turbo muffler or sound canceling system like Flowmasters. That's all an article in itself.
<CENTER>Now for the REAL meat of the issue.</CENTER>
That's all well and good, but why does it affect torque at low revs? Because backpressure restricts velocity. It's the velocity of the air that actually dictates the performance characteristics of the system. High velocity exhaust increases the scavenging effect of the vacuum behind the air mass. The vacuum helps pull the remaining gasses from the combustion chamber.
There's a balance of velocity and flow that has to be taken into consideration. Take this in for instance. You can install a 6" exhaust system on your car. The backpressure will be extremely low. But so will the velocity. The exiting emissions will have a very large tube to travel in, which allows radial travel of the gasses further outward. Thereby reducing the linear force the gas withholds. That intern reduces velocity and the scavenging effect. But, the system has the ability to flow a greater quantity. On the same note, if you have a system with too much backpressure, like a system with a very small tube, the velocity will be very high. But the amount the system can flow will be very low.
The real performance gains happen when the balance between the flow and velocity is in unison. That is, the system with less pressure requires a higher amount of flow to achieve the same velocity as the higher restriction system. Lets take a totally uncalculated example. One engine with a 3" system and an engine with a 1" system. The "magic velocity" for our fake example will be 5m/s. For argument's sake, the 3" engine needs to run at 6,000rpm to achieve an exhaust velocity of 5m/s. Where as the 1" engine only needs to run at about 2,000rpm to achieve 5m/s of exhaust velocity. Keep in mind that this is by no means a calculated example.
The engine in this example has the greatest scavenging effect when the exaust velocity is at 5m/s. The system with the least backpressure (the 3" system) requires more revs to hit 5m/s worth of exhaust velocity. The system with the most backpressure (the 1" system) needs less revs to hit an exhaust velocity od 5m/s.
Each engine has a different "magic number". That is, the velocity of exiting emissions that produces the highest scavenging efficiency. The lower the backpressure; the higher the flow required to reach the desired velocity. The higher the flow required, the higher the rpm required. Thereby defining that a lower backpressure system makes the big torque at higher rpms. And makes less torque at lower revs. But as an overall scenario, the least backpressure makes the most horsepower in the end.
Homepage of Michael R. Lannan, Milwaukee School of Engineering
This is how I've always understood it also....if I'm still wrong, sorry.
BACK PRESSURE
Hey yall. I've been asked this question on a number of occasions. Why does backpressure effect torque? Which is usually accompanied my the generalization that low backpressure reduces torque. Well, I figured an article was in order after the last time it was asked.
Your assumption of the effect of backpressure is the same as most peoples. That is, that having less backpressure decreases torque and increases horsepower. Likewise, having more backpressure increases torque and decreases horsepower.
I've heard this from countless people, and I always try to convince them otherwise, usually to no effect. So I'll try with you. Hopefully I can explain this well enough. The real crux of all this is at the bottom. But read the article carefully. There is a serious flaw in the logic of this assumption about torque. The rule is generalizing on a level that almost seems greedy.
As a general rule that assumption is correct for the lower revs of an engine. That is, if you took a standard engine, and had nothing more than a set of headers on it, you would find that the low-end torque is somewhat less than that of the same engine with some fairly restrictive mufflers. Likewise, the engine's torque, at bigger revs, would be higher than that of the same engine with mufflers. For conversation's sake, I'll consider low-end to be anywhere from 0-1,800rpm, mid-range to be about 1,800-3,000, and high-end to be 3,000rpm and greater.
I'm not sure where people have come up with the idea that there is a counter-productive variation in horsepower by opening up the exhaust system. I've never come across any studies that prove that horsepower is adversely effected by open exhaust. Not that that is what you claimed; but again. I've heard it from several people.
What it all comes down to is what range your looking at. If you want big horsepower and torque; it's going to have to be with an open exhaust. There is no way to really look at it other than that. The more air you can remove, the more air you can take in. More air means more power. And engine with opened exhaust will always make more peak horsepower and torque than a restricted engine. It will just be at a higher revs.
Exhaust modifications are done purely to alter the mass of air that can exit the engine (duh). Here's a story for you. A friend of mine put a new, uncorked, exhaust system on his truck. The engine is an otherwise stock, injected, small block. He put the exhaust on, and his fuel efficiency went way down, torque went down the drain, and his horsepower was basically non-existent. "But I don't get it. I was supposed to get 10hp out of that."
Why did that happen? Because the engine was outside of it's stochiometric ratio. What is that? That's the chemically "neutral" balance of air/fuel ratio. It has been shown that engines produce maximum horsepower at 14.7:1 (air/fuel) by mass. The problem is that when you alter the backpressure of the exhaust system, the ratio will also be altered. If you reduce backpressure, the engine is allowed to flow more air out. More air out means more air in. If the fuel system is not compensated for the increased air mass, the engine runs lean. If the backpressure is increased, the mass of air leaving is decreased. Less air out means less air in. Again, if the fuel system is not calibrated for the change, the engine will run rich.
As far as normal engines are concerned, alterations on the order of 1.3 parts of air to fuel would result in significant loss of performance. And yes, that's loss; no matter which way it goes. Contrary to popular beliefs, lean engines do not produce more horsepower and/or torque than neutral. So taking an engine, tuned for a specific amount of backpressure, and altering it, would result in the engine running outside of its stochiometric range. The engine would thereby run substantially lean, and be running quite poorly.
Now note that I said it was an injected small block. You might be thinking "well doesn't the ECM calibrate itself for the differing air mass?" Yes, it would. To an extent. Once you start getting somewhere in the range of about 1.5:1 outside of neutral, ECMs are typically unresponsive. As a general rule a code would be thrown and the ECM would just swing to it's maximum ability.
To most people's surprise, the "flowability" of an exhaust system is determined more by its exhaust manifold than the mufflers that are installed. Keep in mind that headers, of whatever style, are nothing more than exhaust manifolds. It is possible to have a low restriction exhaust that boosts low-end torque. That is truly what is wanted if you're looking for low-end torque. Since the backpressure isn't the "direct" cause of the situation at hand; but rather an indirect cause. Keep reading and you'll see what I mean.
A common street-stock exhaust system has about 15-20psi of parasitic pressure loss. Compare that to a typical muffled performance system of about 5psi of parasitic loss. A tube header and extension pipe with no baffling or resonators (an open header system) would have about 2psi of loss.
A street-stock system is one in which all the usual stock equipment is on the vehicle. That is a cast style exhaust manifold, reverse-flowing mufflers, and standard diameter tubing with heavy bend kinks. A performance system is one where the engine employs tubular headers, larger exhaust tubing (usually) and a perforated straight-through baffle muffler. Notice I did not say turbo muffler or sound canceling system like Flowmasters. That's all an article in itself.
<CENTER>Now for the REAL meat of the issue.</CENTER>
That's all well and good, but why does it affect torque at low revs? Because backpressure restricts velocity. It's the velocity of the air that actually dictates the performance characteristics of the system. High velocity exhaust increases the scavenging effect of the vacuum behind the air mass. The vacuum helps pull the remaining gasses from the combustion chamber.
There's a balance of velocity and flow that has to be taken into consideration. Take this in for instance. You can install a 6" exhaust system on your car. The backpressure will be extremely low. But so will the velocity. The exiting emissions will have a very large tube to travel in, which allows radial travel of the gasses further outward. Thereby reducing the linear force the gas withholds. That intern reduces velocity and the scavenging effect. But, the system has the ability to flow a greater quantity. On the same note, if you have a system with too much backpressure, like a system with a very small tube, the velocity will be very high. But the amount the system can flow will be very low.
The real performance gains happen when the balance between the flow and velocity is in unison. That is, the system with less pressure requires a higher amount of flow to achieve the same velocity as the higher restriction system. Lets take a totally uncalculated example. One engine with a 3" system and an engine with a 1" system. The "magic velocity" for our fake example will be 5m/s. For argument's sake, the 3" engine needs to run at 6,000rpm to achieve an exhaust velocity of 5m/s. Where as the 1" engine only needs to run at about 2,000rpm to achieve 5m/s of exhaust velocity. Keep in mind that this is by no means a calculated example.
The engine in this example has the greatest scavenging effect when the exaust velocity is at 5m/s. The system with the least backpressure (the 3" system) requires more revs to hit 5m/s worth of exhaust velocity. The system with the most backpressure (the 1" system) needs less revs to hit an exhaust velocity od 5m/s.
Each engine has a different "magic number". That is, the velocity of exiting emissions that produces the highest scavenging efficiency. The lower the backpressure; the higher the flow required to reach the desired velocity. The higher the flow required, the higher the rpm required. Thereby defining that a lower backpressure system makes the big torque at higher rpms. And makes less torque at lower revs. But as an overall scenario, the least backpressure makes the most horsepower in the end.
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